(1) Field of the Invention
The present invention relates to a gain control unit and an optical amplifier for compensating a gain wavelength characteristic for an optical signal, and more particularly to a gain control unit and an optical amplifier having a temperature compensating function which also compensates automatically a temperature dependency of the gain wavelength characteristic.
(2) Description of the Related Art
There has been conventionally known a gain equalizer (gain control unit) utilizing such as an optical filter which is provided within an optical amplifier to be used in an optical communication system, to compensate a gain wavelength characteristic of the optical amplifier. This conventional gain equalizer is designed in advance such that its loss wavelength characteristic compensates the gain wavelength characteristic of the optical amplifier to thereby obtain an overall flat wavelength characteristic. Generally, the design of loss wavelength characteristic of the gain equalizer is mostly optimized on the basis of the gain wavelength characteristic of the optical amplifier at a specific temperature. As such, it has been required that the temperature dependency of the loss wavelength characteristic of the gain equalizer is less. By adopting such a gain equalizer, it has become possible to realize an optical amplifier having a relatively flat gain wavelength characteristic in a predetermined wavelength band.
Meanwhile, it is known that a gain wavelength characteristic of an optical amplifier fluctuates depending on ambient temperature. For example, Erbium-doped fiber (EDF) typically used in an optical amplifier has such a gain wavelength characteristic as shown in FIG. 16, in which the gain deviation relative to the wavelength varies largely depending on the ambient temperature (0.degree. C., 25.degree. C., and 65.degree. C., in this figure). Further, the difference between the gain at the ambient temperature of 0.degree. C. and that at the ambient temperature 25.degree. C. in the above mentioned EDF exhibits such a wavelength characteristic (A) as shown in FIG. 17, and the difference between the gain at the ambient temperature 65.degree. C. and that at the ambient temperature 25.degree. C. exhibits a wavelength characteristic (B) in FIG. 17.
Since a gain wavelength characteristic of an EDF has a temperature dependency as noted above, it has been conventionally required to keep the EDF at a constant temperature. As such, it has been required to cover the EDF such as by a heat insulating material, and occasionally to adjust the temperature of the EDF by a heater or power source and the like, causing a disadvantage of increase in size of an optical amplifier.
Relatedly, there exists a technique such as disclosed in Japanese Unexamined Patent Publication 9-145941, as a long period fiber grating ("LPG", which can be used to flatten a gain wavelength characteristic of EDF) aiming at improving stability against temperature variation. In this technique, there are newly designed a profile of cladding and a composition of fiber, in view of the fact that the refractive indices of core and cladding vary differently from each other relative to temperatures due to a temperature dependency of a long period fiber grating. Contrary, as described later herein, the present invention resides in a technique for realizing temperature compensation by appropriately combining fiber gratings having mutually different characteristics, in which the structures of the fiber gratings are common in themselves. Thus, the present invention is distinguished from the aforementioned known technique.